Control for a dragline



Aug. 12, 196 J. A. PESAVENTO ET AL 3,460,278

CONTROL FOR A DRAGLINE Filed Oct. 21, 1965 RD HOIST DRAG I MOTOR MOTOR R CONTROL CONTROL B GENERATOR US. Cl. 37-116 14 Claims ABSTRACT OF THE DISCLOSURE There is shown a dragline wherein two-axis control of the bucket is provided by a master controller with a single handle operating along intersecting courses one for each axis of control. The controller produces signals which are functions of the positions of the controller along each course. The signals, which represent the vertical and horizontal components of a desired resultant velocity for the bucket, are translated or converted to signals proportional to the velocities required for the hoist and drag cables to effect the desired resultant velocity of the bucket. The latter signals are employed to control the hoist and drag cables.

This invention relates to a semi-automatic control system for a dragline, and more particularly to a control system of the type described in which hoist (up and down) and drag (in and out) directional signals for the dragline are obtained from a single master controller with two courses of movement at an angle to each other, for example around intersecting axes.

As is known, a dragline includes a bucket suspended from an inclined boom by means of a cable, the bucket 'being designed such that when it is pulled toward the base of the boom by a second cable, it will scrape soil or other material from the surface being excavated. The aforesaid first and second cables are normally wound upon motordriven reels such that rotation of the respective reels in one direction or the other will determine the position of the bucket and its direction of movement.

In the past, the operation of such draglines has required a great deal of physical coordination on the part of the operator. The reason for this is that in the conventional dragline control, one lever determines the up and down movement of the bucket (i.e., along the vertical or y-axis); while another lever controls the in and out movement of the bucket along a horizontal path (i.e., the x-axis). In order to manipulate the bucket, both levers must be actuated simultaneously, meaning that the operation of a dragline takes a great deal of skill and requires that a new operator receive considerable training before he can operate the equipment efficiently.

As an overall object, the present invention seeks to provide a new and improved system for operating a dragline, which system eliminates much of the physical coordination and skill required of the operator by prior-art control systems for such equipment.

More specifically, an object of the invention is to pro vide a control for a dragline in which the hoist and drag directional signals for the dragline bucket are derived from a single master switch having two courses of movement. In this manner, instead of manipulating two levers as in prior-art control systems, the operator simply has a single lever which he can move toward and away from himself and also up or down. In this respect, the lever is 3,460,278 Patented Aug. 12, 1969 mounted on what can be compared to a ball and socket joint. In order to pull the bucket toward the base of the boom along a horizontal path, the operator simply pulls the levertoward himself. Conversely, to move the bucket away from the base of the boom along a horizontal axis, he will simply push the lever away from himself. Up and down movement of the backet is controlled by moving the lever up or down, as the case may be. The control system is such that for any position of the control lever, the reels on which the cables are wound are automatically rotated in one direction or the other to position the bucket as demanded by the control lever.

Another object of the invention is to provide a control system of the type described above in which movement of the two course control lever produces signals which are proportional to the desired velocity components of the dragline bucket. In this way, the further the operator moves the control lever from its central or null position, the faster will be the velocity of the bucket in the desired direction.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying single figure drawing which schematically illustrates one embodiment of the control system of the invention.

With reference now to the drawing, a dragline is shown including a housing 10 mounted for pivotal movement on a base or tub 12, and adapted to move over the ground by means of crank-driven shoes 14. Projecting outwardly from the housing 10 is an inclined boom 16 having a pulley 18 at its forward, upper end. A first cable 20 is wound upon a reel 22 in housing 10; passes around pulley 17 mounted on housing 10 and pulley 18, and is connected to the top of the dragline bucket 24. A second cable 26 is wound upon reel 28 in housing 10 and is connected, as shown, to the forward upper edge of the dragline bucket 24. Thus, clockwise rotation of reel 28, for example, will cause the bucket 24 to move toward the housing 10; while rotation of reel 22 in one direction or the other will cause the bucket 24 to move up or down, as the case may be. Reel 22 is connected to a reversible hoist drive motor 34 through mechanical gearing schematically represented in the drawing by broken line 36. Similarly, the reel 28 is connected to drag drive motor 30 through mechanical gearing represented by broken line 32. The motors 34 and 30 are controlled by suitable motor control circuits 38 and 40, respectively.

From the drawing, it can be seen that the cables 20 and 26 define a triangle in which:

0, O=Origin of horizontal and vertical reference system.

B=Length of boom 16 between point 0, O and point R Length of cable 20 between point x and point x, y (i.e., length proportional to active cable removed from hoist drum 22).

R =Length of cable 26 between point x, y and point 0, O (i.e., length proportional to active cable removed from drum 28).

0 0 0 =Associated included angles B-R -R =Angle between horizontal and R =Constant angle determined between dimension B and horizontal at point 0, O.

0 :Constant angle determined by B and vertical at point B YB- ot=Angle between vertical and R at point x, y.

of triangle (Equation III) ittl hd fi hl and R =Rate of change of length of R R =Rate of change of length of R a3=Rate of change of position of x coordinate of bucket;

and

qj=Rate of change of position of y coordinate of bucket.

where Again, from the geometry of the triangle shown in the drawing:

Therefore, Equations III and IV become: (Equation V) R =(a'2 sin 04+!) cos or) and (Equation VI) R =(zt cos tP-H] sin 1,0) Thus, by developing signals proportional to the desired rate of change of the bucket along the x-axis (i.e., at) and along the y-axis (i.e., 17), command signals R and R for driving motors 30 and 34 and, hence, reels 22 and 28 can be derived, assuming that the angles ,0 and t can also be determined.

Circuitry for accomplishing the foregoing is shown in the drawing and includes a rheostat 46 connected through gear reducer 47 (represented by broken line) to drum 22 for producing a first alternating current electrical signal on lead 48 proportional in magnitude to the length, R of cable 20 between points x, y and x 35;. A second rheostat 42 is connected through gear reducer 43 (represented by broken line) to reel 28 and is adapted to produce an alternating current electrical signal on lead 44 proportional in magnitude to the length, R of cable 26 between point 0, O and point x, y. An alternating current signal generator 50 is arranged to produce a signal on lead 52 which is proportional in magnitude to the distance, B between points 0, O and x 31 this signal being of constant magnitude.

The signal proportional to B on lead 52 is applied through a first squaring multiplier 54, the output of multiplier 54 being inverted in inverter 56'to produce the signal -B In a similar manner, the signal proportional to R on lead 48 and the signal proportional to R on lead 44 are passed through squaring multipliers 58 and 69, respectively. The signal at the output of multiplier 58, is, therefore, R The signal at the output of multiplier 60 is inverted in inverter 62 to produce the signal -R The signals --B R and R are applied to an operational amplifier 64 which adds them to produce an electrical quantity equal to R B R Reverting again to Equation I given above, it can be written as:

It can be seen, therefore, that the inputs R B and R comprise one-half of the equation given above. Therefore, if a fourth input comprising 2BR cos 6 is applied to the fourth input lead 66 for the operational amplifier 64, all of the input signals will balance each other and the output of the operational amplifier 64 on lead 68 will be zero.

The output from the operational am lifier 64 is applied to a servomotor 70 which will be caused to rotate in one direction or the other, depending upon the output signal on lead 68. The servomotor 70, in turn, rotates the rotor of a resolver, generally indicated by the reference numeral 72. Resolver 72 may, for example, be of the type manufactured by the Ford instrument Company, Long Island City, N.Y. and includes a pair of windings 74 and 76 wound at right angles to each other on a rotor element connected through linkage 78 to the servomotor 70. One or more stator windings are included in the resolver 72, only one winding 30 being utilized in the present instance.

The basic operation of a resolver is exemplified by its computation of the sine and cosine of an angle. For this computation, the stator winding 80 is supplied with a variable alternating current voltage proportional to ZBR In the present case this voltage, proportional to ZBR s derived by multiplying the signal proportional to B by the signal proportional to R in multiplier 82 to roduce an output signal proportional to BR Two signals proportional to BR are summed in adder 84 to produce the signal ZBR When the servomotor 70 rotates, the shaft 78 is turned to represent a particular angle, and the two rotor windings 74 and 76 provide output voltages that are proportional to the product of the signal applied to stator winding 80 times the sine and cosine of the angle to which the shaft 78 was turned. In the present instance, only the one rotor winding 74 is employed, which produces an output signal on lead 66 equal to ZBR cos 0 Thus, the output signal on lead 66 is dependent upon the magnitude of the voltage ZBR and the degrees of rotation of shaft 78.

As long as the quantity ZBR cos 6 is equal to R "B R in accordance with Equation I given above, the output of the operational amplifier 64 appearing on lead 68 will be zero and the servomotor 70 will not turn. When, however, the system becomes unbalanced as when the bucket 24 is moved from a previously-established osition, the servomotor 70 will rotate in one direction or the other until the system is again brought back into balance.

The shaft 78 of servomotor 70 is also connected to one gear 86 of a mechanical differential device 88. The differential device 88 includes two beveled gears 90 and 92 rotatable about shafts which are at right angles with respect to the shaft 78. The gear 92 is connected to a dial 94 which is adjusted for the angle 0, this angle being fixed since the position of the boom 16 is fixed with respect to horizontal. The operation of the differential device 88 is such that the degrees of rotation of bevel gear 90 will be equal to the degrees of rotation of gear 86 minus the degrees of rotation effected by the dial 94. Since the angular position of shaft 78 is equal to 0 and since 0 is equal to given above, it will be appreciated that subtraction of from +ip gives the angle 1p. Thus, the

angular rotation of gear 90 is equal to b; and since the gear 90 is connected to the rotor of a second resolver 96 through coupling 98, the rotation of the rotor of resolver 96 will be a number of degrees equal to 0.

The rotor of resolver 96 again includes two windings 100 and 102 at right angles to each other, only the winding 102 being utilized in the present instance. In this case, however, the two stator windings 104 and 106 are energized by signals ab and respectively. The manner in which at and y are derived will be described hereinafter. When a resolver is provided with two input windings such as the resolver 96, the output on rotor winding 102, for example, will be equal to the magnitude of the input signal on one stator times the cosine of the angle representing the angular position of the rotor, plus the magnitude of the other input signal times the sine of the angle. Thus, in the case of resolver 96, the output on winding 102 will be (ico -l-U sin ,0). Reverting, again, to Equation VI given above, it will be seen that this is the required control signal proportional to R for motor 30.

The signals X and y are derived from a two-axis master controller, under the control of the operator, and identified generally by the reference numeral 108. It includes a generally horizontal handle 110 (shown vertical in the drawing for purposes of explanation) secured to a shaft 112 mounted for rotation on a ring member 114. The ring member 114, in turn, is mounted for rotation on shafts 116 and 118. In this manner, up and down move ment of the handle 110 will cause the shaft 112 to rotate; while movement of the handle forward (out) or reverse (in) will cause shafts 116 and 118 to rotate.

The shaft 112 is connected to the wiper brush of a rheostat 120 having its opposite ends connected to a source of alternating current voltage 121. A center tap on the rheostat 120 is grounded, as shown. Thus, when the handle 110 is in its center position, zero output voltage will be applied from source 121 to winding 106- of resolver 96. However, it will be appreciated that the voltage from the alternating current source 121 applied to winding 106 changes in magnitude as the handle 110 is moved further away from its center position, either upwardly or downwardly. As the handle 110 moves upwardly, the signal R increases at the input to control circuit 38 in a manner hereinafter described to cause the circuit 38 to rotate motor 34 in one direction; whereas when the handle 110 is moved downwardly, the phase of the signal R is reversed in phase such that it causes the control circuit 38 to rotate motor 34 in the other direction. The speed of motor 34 is determined by the magnitude of the signal R derived from winding 102. Consequently, the speed of the bucket 24, up or down, will be dependent upon the amount of movement of handle 110 up or down from its center position. For instance, if the operator Wishes to move the bucket 24' upwardly at a faster rate, he will simply push the handle 110 upwardly to a further extent from its center position.

It will be noted that up or down movement of the bucket is determined by the phase of the signal R which phase is determined by the position of the center tap of rheostat 120 on either side of its grounded center point. A direct current directional output in the motor control circuit 38 is derived from the alternating current signal R in accordance with conventional techniques through the use of a phase detecting AC/DC transistor chopper demodulator, filter and amplifier, not shown, to produce a plus or minus 20 watt direct current signal of about 40 milliamperes. In this way, it will be appreciated that when the wiper brush of rheostat 120 is on one side of the grounded center tap, the phase of signal R will produce a direct current signal of one polarity at the output of motor control circuit 38 which causes motor 34 to rotate in one direction. The speed of the motor, as explained above, is dependent upon the amount of movement of control handle 110 and, hence, the distance of the wiper brush of rheostat 120 from its grounded center tap. Similarly, when the wiper brush of rheostat moves to the other side of the grounded center tap, its phase is shifted by 180 such that the chopper, demodulator, filter and amplifier combination described above will produce a direct current signal of the opposite polarity which causes motor 34 to rotate in the opposite direction at a speed determined by the displacement of the wiper brush from the grounded center tap of rheostat 120.

Movement of bucket 24 along the x-axis is affected in a somewhat similar manner. That is, the shaft of master controller 108 is connected to the wiper brush of a second rheostat 126 energized by source 127 and having a grounded center tap. In this manner, forward (out) or backward (in) movement of the handle 110 will cause the alternating current voltage applied to winding 104 to change in magnitude, the further the handle 110 is moved fromv its center position, the greater the magnitude of the alternating current signal. Furthermore, when the center tap of rheostat 126 is on one side of the grounded center tap, the signal applied to winding 104 will be 180 out of phase with respect to that when the Wiper brush is on the other side of the center tap.

Reverting, again, to the resolver 96, the signal on winding 102 will be equal to the magnitude of the signal (ab cos 0+1] sin 1,11). In accordance with Equation VI given above, this is equal to the desired control signal R for motor control circuit 40. Assuming that the control handle 110 is in its center position as shown, the output signal R will be zero and the motor 30 and reel 28 will be stationary. When, however, the handle 110 is pulled in reverse, for example, a signal will be produced on winding 102 of resolver 96 to cause the motor 30 and reel 28 to rotate in a clockwise direction, thereby causing the bucket 24 to move toward the housing 10'. When this occurs, the signal R produced by rheostat 42 will vary, thereby varying the signals applied to the operational amplifier 64 and causing servomotor 70' to rotate such that the angular position of shaft 98 always corresponds to the angle Ill. Of course, the further the handle 110 is pulled, the faster will be the motion of the bucket 24 toward the housing 10, as was explained above.

To this point, only the control circuitry for motor 30 has been described. The control circuitry for motor 34 is similar and includes a squaring multiplier 132 which produces an output applied through inverter 134 to produce the signal B The signal R is applied through squaring multiplier 136 and inverter 138 to produce the signal R The signal R is applied through squaring multiplier 140 to produce the signal R and the three signals R R -B are applied to an operational amplifier 142 along with a fourth signal derived from a rotor winding 144 on resolver 145. As will be understood, the signals R R and B could be derived from multipliers 54, 58 and 60; however, separate multipliers are shown herein for purposes of illustration. The stator Winding 146 of resolver has applied thereto a signal proportional to ZBR as is derived from multiplier 148 and adder 150 in the manner described above in connection with circuits 82 and 84. Thus, the signal on lead 152 from rotor winding 144 will be proportional to ZBR cos 0 to satisfy Equation II given above.

The output of the operational amplifier 142 is applied to a servomotor 154 similar to servomotor 70 described above. The shaft 156 of servomotor 154 is connected to the rotor of resolver 145 and also to a first bevel gear 158 of a second differential 160. Gear 162 of differential is connected to a dial 164 which, in this case, is rotated to an angle corresponding to 0 this angle being that between vertical and the dimension B, and is fixed for a particular operating condition of the dragline due to the fact that the position of the boom 16 is fixed.

From the foregoing equations, it will be appreciated that rotation of the third gear 166 of differential 160 will represent the angle a. The gear 166 is connected through shaft 168 to the rotor of a resolver 170. Applied to the stator windings 172 and 174 of resolver 170 are the signals 7] and a; from rheostats 120 and 126, respectively. In this case, the output signal R is derived from one of the two rotor windings 176 such that the output is represented by (a'e sin u+y cos Thus, the signal R satisfies Equation V given above.

As the control handle 110 is moved forward or in reverse, and up or down, control signals R and R will be produced to rotate reels 28 and 22 so as to manipulate the bucket 24 as desired by the operator. The speed at which the bucket 24 moves along the x and y axes is, as mentioned above, determined by the amount of movement of the handle 110 from its center position forward or reverse and up or down. Thus, in order to move the bucket 24 faster, the operator will simply move the handle 110 further from its center position in either plane.

Should the operator, for example, desire to move the bucket 24 toward housing 10 along a horizontal path, it will be necessary for reel 28 to rotate in a clockwise direction and for reel 22 to also rotate in a clockwise direction. If reel 22 were not to rotate, the dimension R would remain the same and the bucket 24 would begin to elevate rather than remain on a horizontal path, as is desired. Therefore, as the operator pulls back on handle 110, the signal y will be changed thereby varying the output of both resolvers 96 and 170 (i.e., signals R and R These changes will cause both reels to rotate in clockwise directions; but at the same time, the quantities R and R will be changing to bring the system back into balance through servomotors 70 and 154.

Similarly, if the operator desires to elevate the bucket 24 along a vertical path, he will push the handle 110 upwardly, thereby changing the position of the wiper brush on rheostat 126. This causes the reel 22 to rotate in a counterclockwise direction; but at the same time the reel 28 must rotate in a clockwise direction since the di mension R must be shortened under the circumstances described. Movement of the reel 28, of course, is elfected by virtue of the fact that the signal on winding 174 of resolver 170 is varied at this time to also vary the signal R Combined movement of bucket 24 along the x and y axes can, of course, be effected by rotating the handle 110 simultaneously about shafts 116 and 112. In all cases, however, the foregoing equations will be satisfied by the circuitry shown in the drawing, and the velocity of the bucket along either axis will be dependent upon the amount of rotation of the shafts 116 and 112 from their center positions.

While the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. In a control system for material handling apparatus in which a material handling device is suspended from a boom by a cable wound upon a first reversible motordriven reel and wherein the device is pulled toward the base of the boom by a cable wound upon a second reversible motor-driven reel; the improvement comprising a single master controller movable along a first course and along a second course at an angle to the first course for controlling vertical and horizontal movement of the device respectively, means for producing a first electrical signal which varies as a function of the position of the controller along said first course, means for producing a second electrical signal which varies as a function of the position of the controller along the second course, and circuitry connected to said controller and responsive to said first and second signals for controlling said motordriven reels and the position of said device, said circuitry including a first servo system for controlling one of said motor-driven reels and a second servo system for con trolling the other of said motor-driven reels, each of said servo systems incorporating a mechanical differential device including a plurality of intermeshed gears, at least two of said gears being connected to respective electromechanical resolvers and one of said gears being connected to a servomotor within its associated servo system.

2. In a control system for material handling apparatus in which a material handling device is suspended from a boom by a first cable wound upon a first reversible motordriven reel and wherein said device is pulled toward the base of the boom by a second cable wound upon a second reversible motor-driven reel; the improvement of means for developing an electrical signal R for controlling rotation of said first reel in accordance with the equation:

R (at sin a-l-y cos a) where a: is the desired rate of change of said device along horizontal, y is the desired rate of change of said device along vertical, and on is the angle between said first cable as suspended from the end of the' boom and vertical, means for developing an electrical signal R for controlling rotation of said second reel in accordance with the equation:

R :(-6i cos 0+ sin ,b)

where a is the angle between said second cable and horizontal, means responsive to the signal R for controlling rotation of said first reel, and means responsive to the signal R for controlling rotation of said second reel.

3. The combination as in claim 15 wherein there is means for producing two signals respectively proportional to 41'; and 1], and means for producing three signals B, R and R respectively proportional to the lengths of the sides of the triangle defined by said boom and the active lengths of said first and second cables, and wherein said means for developing the signal R performs that development in response to said signals 02", 17, R R and B, and wherein said means for developing the signal R performs that development in response to said signals 02, 1], R R and B.

4. The combination as in claim 3 wherein said material handling apparatus is a dragline, and said material handling device is a bucket.

5. The combination as in claim 2 wherein said material handling apparatus is a dragline, and said material handling device is a bucket.

6. In a control system for material handling apparatus in which a material handling device is suspended from a boom by a first cable wound upon a first reversible motordriven reel and wherein said device is pulled toward the base of the boom by a second cable wound upon a second reversible motor-driven reel; the combination of means for developing electrical signals B, R and R which are proportional to the lengths of the sides of the triangle defined by said boom and the active lengths of said first and second cables, means for producing an electrical signal proportional to ZBR means including apparatus responsive to the electrical signals proportional to R R B and ZBR for developing an electrical signal R for controlling the rotation of said first reel in acccordance with the equation:

R (isin a-l-y' cos at) where at is the desired rate of change of said device along horizontal, y is the desired rate of change of said device along vertical, and 0c is the angle between said first cable as suspendd from the end of the boom and vertical, means for developing an electrical signal proportional to ZBR means including apparatus responsive to the electrical signals proportional to R R E and ZBR for developing an electrical signal R for controlling rotation of said second reel in accordance with the' equation:

R (02 cos .9+ sin 1/) P? where #1 is the angle between said second cable and horizontal, means responsive to the signal R for controlling rotation of said first reel, and means responsive to the signal R for controlling rotation of said second reel.

7. The combination of claim 6 whrein the means for developing the signal R includes a device for varying R as a function of the angle between the boom and horizontal, and wherein the means for developing the signal R includes a device for varying R as a function of the angle between the boom and vertical.

8. The combination as in claim 6 wherein said material handling apparatus is a dragline, and said material handling device is a bucket.

9. In a control system for material handling apparatus in which a material handling device is suspended from a boom by a first cable wound upon a first reversible motordriven reel and wherein said device is pulled toward the base of the boom by a second cable wound upon a second reversible motor-driven reel; the improvement of means for generating three electrical signals proportional to B R and R wherein B, R and R are proportional in magnitude to the lengths of the sides of the triangle defined by said boom and the active lengths of said first and second cables, means for producing an electrical signal proportional to 2BR cos where 6 is the angle between the boom and the active length of said second cable, means including apparatus responsive to the signal proportional to R R B and ZBR cos 0 for controlling rotation of said second motor-driven reel, means for producing an electrical signal proportional to ZBR cos 0 where 0 is the angle between said boom and the active length of said first cable, and means including apparatus responsive to the signals proportional to R R B and 2BR cos 0 for controlling rotation of said first motordriven reel.

10. The combination as in claim 9 wherein said material handling apparatus is a dra-gl-ine, and said material handling device is a bucket.

11. In a control system for material handling apparatus in which a material handling device is suspended from a boom by a first cable wound upon a first reversible motordriven reel and wherein said device is pulled toward the base of the boom by a second cable wound upon a second reversible motor-driven reel; the improvement comprising a single master controller movable along a first course and along a second course at an angle to the first course for controlling combined vertical and horizontal movement of said device, means for producing a first signal which varies as a function of the position of the controller along said first course, means for producing a second signal which varies as a function of the position of the controller along the second course, said first and second signals being proportional to the vertical and horizontal components respectively of a desired resultant velocity for said device, circuit means coupled to said master controller for converting said first and second signals to a third signal proportional to that component of said desired resultant velocity which is in line with the first cable, circuit means coupled to said master controller for converting said first and second signals to a fourth signal proportional to that component of said desired resultant velocity which is in line with the second cable, means for controlling the first motondriven reel in response to said third signal, and means for controlling the second motor-driven reel in response to said fourth signal.

12. The combination as in claim 11 wherein said material handling apparatus is a dragline, and said material handling device is a bucket.

13. In a control system for material handling apparatus in which a material handling device is suspended from a boom by a first cable wound upon a first reversible motor-driven reel and wherein the device is pulled toward the base of the boom by a second cable wound upon a second reversible motor-driven reel: the improvement comprising means for providing signals 1] and a? proportional to desired vertical and horizontal components respectively of a desired resultant velocity for said device, means for generating three signals B, R and R which are respectively proportional in magnitude to the lengths of the sides of the triangle defined by said boom and the active lengths of said first and second cables, means responsive to said signals 1], at, B, R and R for controlling rotation of the first motor-driven reel in accordance with that component of said desired resultant velocity that is in line with the first cable, and means responsive to said signals 1], at, B, R and R for controlling rotation of the second motor-driven reel in accordance with that component of said desired resultant velocity that is in line with the second cable.

14. The combination as in claim 13 wherein said material handling apparatus is a dragline, and said material handling device is a bucket.

References Cited UNITED STATES PATENTS 2,633,649 4/1953 Page 37116 2,978,820 4/1961 Johnson 371 16 3,084,805 4/ 1963 McKinnon.

3,144,146 8/1964 Strickland 214- ANTONIO F. GUIDA, Primary Examiner A. P. KOPECKI, Assistant Examiner US. Cl. X.R. 214-657 23 3 33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 2 Datefi p 3-, 1969 Inventor) Joseph A Pesavento and Darl C. washburn, Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' Column 8, line 31, the parent claim reference numeral "15" should read 2 SIGNED AN'U SEALED DEC 9 m9 (SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E, SCHUYLER, JR-

Attesting Officer C-onmissioner of Patent. 

